Pseudomorphic HEMTs (high electron mobility transistors)--abbreviated PHEMT in the following--grown on a GaAs substrate are field effect transistors with a braced channel layer, in which the doping atoms that provide the charge carriers required for the charge transport from the source to the drain are wholly or partially spatially separated from the channel in which the charge transport takes place. The construction of the semiconductor layer structure required therefor is grown epitaxially. The channel, which standardly consists for example of In.sub.y Ga.sub.1-y As, thereby has the following characteristics:
1. The lattice constant of the semiconductor crystal in the channel is (dependent on the In content y) different from the lattice constants of the surrounding semiconductor materials and of the substrate (thus the designation "pseudomorphic"). Thus, the thickness of the channel is limited if displacements, which can occur due to the insufficient adaptation of the lattice constants and which have an adverse effect on the charge carrier transport characteristics, are to be avoided.
2. The transport characteristics of the charge carriers are better in the channel than in the adjacent semiconductor layers, which likewise depends on the In portion y.
3. The energy band gap is smaller in the channel than in the adjacent layers. For this reason, the charge carriers are transferred from the doped layers adjacent to the channel into the channel, and are effectively enclosed there, i.e. in the region with the best transport characteristics. However, the energy band gap of the channel cannot be reduced arbitrarily, because very small band gaps can be achieved only with In portions y that cause sharp deviations of the lattice constants from that of the substrate or, respectively, of the remaining semiconductor layers, and, consequently, enable only very small thicknesses of the channel.
In the design of epitaxial layers for PHEMTs, a compromise must thus be found. Contents y of In that were selected with respect to a minimum band gap allow only channel thicknesses small enough that the number of charge carriers enclosed in the channel is very small, and their transport characteristics are strongly adversely influenced by the channel boundary surfaces lying close to one another. Channels that have been optimized on one side to a large thickness have band gaps that differ only slightly from those of the adjacent layers, so that the charge carriers are no longer transferred effectively from the doped layers into this channel, and are enclosed there only defectively. Moreover, in these channels the transport characteristics are only slightly superior to those of the adjacent layers.
PHEMTs on the basis of GaAs have been used up to now in two basic forms, known as single heterojunction PHEMT (SH-PHEMT) and double heterojunction PHEMT (DH-PHEMT). Both basic forms have a channel in common, which, given In portions y of 0.2 to 0.25, is typically 10 nm to 12 nm thick.
In the SH-PHEMT, the In.sub.y Ga.sub.1-y As channel is bounded below by a GaAs layer, and above by a semiconductor material that comprises a greater difference of the energy band gap to InGaAs than GaAs, standardly by means of Al.sub.x Ga.sub.1-x As, more rarely by means of In.sub.z Ga.sub.1-z P. In the SH-PHEMT, doping atoms are located only in the layer above the channel, i.e. between the channel and the surface of the component or, respectively, between the channel and the gate contact, but not in the GaAs under the channel. This means that the lower boundary of the conduction band runs in the direction perpendicular to the boundary surface between the semiconductor material and the gate contact, as shown in FIGS. 2 and 3. Given gate voltages close to the cutoff voltage of the transistor, the conduction band edge runs in the channel almost parallel to the Fermi energy, by which means the location probability of the charge carriers is greatest approximately in the center of the channel (see FIG. 2). Given more positive gate voltages, in the upper part of the channel (i.e., further left in the Figure) the conduction band lies significantly further below the Fermi energy than in the lower part of the channel, which displaces the location probability of the charge carriers upward, toward the gate contact (see FIG. 3). The average distance of the charge carriers to the gate contact is thus reduced, and their controllability by the gate contact, known as the steepness, improves in inverse proportion to this distance. However, more strongly positive gate voltages then also bring the conduction band edge of the doped material between the gate contact and the channel below the Fermi energy, so that here as well there arises an occupation with charge carriers that have poor transport characteristics and that cause the steepness to go down again.
In the DH-PHEMT, the InGaAs channel is bounded on both sides by a doped AlGaAs layer, so that the jump of the energy band edge is of equal height at the upper side and the lower side of the channel (and, as in the SH-PHEMT, is at the upper side only). The resulting path of the conduction band edge, perpendicular to the surface of the semiconductor material, is shown in FIGS. 4 and 5. Given gate voltages close to the cutoff voltage of the transistor, the upper part of the channel (i.e. in the direction toward the gate contact) lies far enough above the Fermi energy that the location probability of the charge carriers is greatest in the lower part of the channel (see FIG. 4). Here they are located close to the boundary surface of the channel with the lower AlGaAs layer, so that boundary surface scatter has a negative influence on the transport characteristics. Given a more positive gate voltage, the conduction band is located relative to the Fermi energy in such a way that now the location probability of the charge carriers is greatest in the center of the channel (see FIG. 5). The distance of the charge carrier to the gate contact has thus here as well become smaller, and the steepness increases, but not as strongly as in the SH-PHEMT (because the distance remains absolutely greater). Still more positive gate voltages bring the conduction band edge of the doped material between the gate contact and the channel into the vicinity of the Fermi energy, so that the steepness decreases again. However, in the DH-PHEMT this effect occurs later than in the SH-PHEMT, so that the range of the gate voltage within which no undesired conduction occurs in the semiconductor material between the gate contact and the channel is larger as a whole.
Given small currents (i.e., small gate-source capacitances), such as must be set in order to achieve a good signal-noise ratio, in the SH-PHEMT the majority of the charge carriers are at a distance from the boundary surfaces of the channel, so that minimal boundary surface scatter occurs, and the signal-noise ratio is superior to that of the DH-PHEMT. The maximum steepness is likewise better than in the DH-PHEMT. The SH-PHEMT is thus on the one hand particularly well suited for low-noise receiving amplifiers, but on the other hand is also well suited for all other small-signal amplifiers up into the highest frequency range (approx. 100 GHz), where the highest steepnesses are required in order to be able to achieve gain at all.
Due to the high discontinuities of the energy band edges at the upper and lower boundaries of the channel, the DH-PHEMT has a larger effective channel cross-section, and, in contrast to the SH-PHEMT, doping atoms can also be used in the semiconductor material under the channel, i.e. on the side of the channel facing away from the gate contact, so that the number of charge carriers flowing in the channel can be greater by about 20% than in the SH-PHEMT. Moreover, the greater discontinuity of the conduction band at the lower boundary of the channel means that the charge carriers require higher energies than in the SH-PHEMT in order to overcome these barriers. Thus, higher drain voltages can be applied to the component, without its being the case that the charge carriers leave the channel and penetrate into deeper layers of the semiconductor crystal, where they can no longer be controlled by the gate voltage. The high number of charge carriers in the channel, in connection with the possible high drain voltages, make DH-PHEMTs particularly suitable for power transistor applications. However, the maximum achievable steepness in the DH-PHEMT is smaller than in the SH-PHEMT, and the noise characteristics are somewhat worse. However, the higher usable range of the gate voltage in comparison to the SH-PHEMT makes the DH-PHEMT suitable for amplifiers in which the noise behavior is not so important, and linearity with the greatest possible dynamic of the input signal is more important.
In the article by M. Wojtowicz et al. in IEEE Electron Device Left. 15, 477-479 (1994), an HEMT on an InP substrate in the material system InGaAs/InAlAs is specified in which the In content in the channel varies continuously. A doping of semiconductor material for the conductivity type provided for the channel is provided only between the gate contact and the channel. As in other SH-PHEMTs, here as well in each operating state the lower edge of the conduction band runs so as to decline in the direction toward the gate contact (cf. FIGS. 2 and 3). In this graduated composition of the semiconductor material in the channel, the average location probability of the electrons is displaced more strongly in the direction toward the gate contact. By this means, the characteristics typical for SH-PHEMTs are amplified. The advantage of the path of the conduction band edge in the graduated composition of the channel is based only on the fact that due to the partially decreased In content (and the thereby reduced bracing), an overall thicker channel is enabled for the transport of charge carriers, which reduces the scatter of the electrons at the boundary surfaces in relation to conventional structures.
In the article by Tae-Kyung Yoo et al. in Appl. Phys. Left 61, 1942-1944 (1992), an HEMT is specified in which the In content of the InGaAs composition of the channel is greatest in the center of the channel. The layer structure is grown on a GaAs substrate. The non-homogenous composition of the channel is supposed to effect only a one-sided optimization of the transistor characteristics with respect to low noise, since the scatter of the charge carriers is reduced at the upper and lower boundary surfaces.